The present invention relates to a steam reformer which generates hydrogen-rich reformed gas by conducting self-oxidation and reforming of a raw material gas-gas in the presence of steam and oxygen.
There is known a steam reformer which generates hydrogen-rich reformed gas by conducting steam reforming of a mixture of raw material gas-gas and steam, (hereinafter referred to as the “raw material gas-steam mixture”), in the presence of a steam reforming catalyst. The hydrogen-rich reformed gas produced in the steam reformer is favorably used as the fuel of fuel cells. The applicable raw material gas-gas includes hydrocarbons such as methane, aliphatic alcohols such as methanol, and ethers such as dimethylether.
The reaction formula of steam reforming in a steam reformer when methane is used as the raw material gas-gas is written as CH4+2H2O→CO2+4H2, where a preferable range of reforming reaction temperature is from 700° C. to 750° C.
Internal heating is a system for supplying heat necessary for the reaction in the steam reformer. The internal heating steam reformer has a partial oxidation reaction bed at the supply side of the raw material gas-steam mixture, (or the upstream side of the reformer). The heat generated in the partial oxidation reaction bed is used to heat the steam reforming bed located at downstream side of the reformer up to the steam reforming reaction temperature. The steam reforming is carried out in thus heated steam reforming catalyst bed to generate the hydrogen-rich reformed gas. The partial oxidation reaction is written as CH4+½O2→CO+2H2, where a preferable temperature for the partial oxidation reaction is 250° C. or above.
An improved model of the internal heating steam reformer is a self-oxidation internal heating steam reformer, and an example thereof is disclosed by Japanese Patent Application Laid-Open No. 2001-192201. The technology according to the patent publication generates heat in the oxidation reaction and the steam reforming reaction in a mixed catalyst bed prepared by mixing an oxidation catalyst and a steam reforming catalyst, respectively, at the same time.
FIG. 13 is a schematic cross sectional drawing of an example of steam reformer of self-oxidation internal heating type developed by the inventors of the present invention. The steam reformer 1 has an inner cylinder 2 and an outer cylinder 3 surrounding the inner cylinder 2. The inner cylinder 2 contains a high-temperature reaction section 6 at the uppermost portion of inside thereof. The high-temperature reaction section 6 contains a mixed catalyst bed 4 prepared by mixing a steam reforming catalyst and an oxidation catalyst, and has an oxygen-containing gas introduction section 5. Beneath the high-temperature reaction section 6, an adjacent section 7 constructed by a heat transfer bed is located. Beneath the adjacent section 7, a high-temperature shift catalyst bed 8 and a low-temperature shift catalyst bed 9 are located in this order.
The steam reforming catalyst is a catalyst for steam-reforming a raw material gas-gas. The applicable steam reforming catalyst includes a Ni-based reforming catalyst such as NiO—SiO2.Al2O3, and a reforming catalyst such as WO2—SiO2.Al2O3 and NiO—WO2.SiO2.Al2O3.
The oxidation catalyst is a catalyst for oxidizing a raw material gas-gas in the raw material gas-steam mixture to generate heat, thereby attaining a temperature necessary for steam reforming. The applicable oxidation catalyst includes platinum (Pt) and palladium (Pd). The mixing ratio of the oxidation catalyst to the steam reforming catalyst is determined within a range from about 1 to 15% depending on the kind of the raw material gas-gas being subjected to steam-reforming. For example, about 5±2% of the mixing ratio is adopted for the case of methane as the raw material gas- and about 3±1% for the case of methanol as the raw material.
The shift catalyst for forming the high-temperature shift catalyst bed 7 and the low-temperature shift catalyst bed 9 includes a mixture containing CuO—ZnO2, Fe2O3, Fe3O4, copper oxide, and the like. If, however, the reaction is carried out at 700° C. or higher temperature, Cr2O3 may be adopted.
The heat transfer bed structuring the adjacent section 7 absorbs heat from the reformed gas leaving the high-temperature reaction section 6 to cool the reformed gas. The heat transfer bed is structured by packing particles such as ceramics particles having good thermal conductivity.
The heat transfer bed may be eliminated in some cases. In that case, for example, the high-temperature shift catalyst bed 8 or both the high-temperature shift catalyst bed 8 and the low-temperature shift catalyst bed 9 structure the adjacent section 7 of the present invention.
Bottoms of the mixed catalyst bed 4, the adjacent section 7, the high-temperature shift catalyst bed 8, and the low-temperature shift catalyst bed 9 located in the inner cylinder 2 are supported by their respective air-permeable supports 10, 11, 12, and 13.
The oxygen-containing gas introduction section 5 has an introduction conduit 14 and ejection part 15 15 opened near the front end of the introduction conduit 14. The oxygen-containing gas may be air or oxygen gas. For example, compressed air supplied from an air compressor (not shown) is supplied to the introduction conduit 14, and the compressed air can be ejected into the mixed catalyst bed 4 through the ejection part 15 15.
A steam reforming catalyst bed 16 is positioned at the uppermost portion of inside the outer cylinder 3, and a heat transfer bed 17 is positioned beneath the steam reforming catalyst bed 16. Bottoms of the steam reforming catalyst bed 16 and the heat transfer bed 17 are supported by their respective air-permeable supports 18 and 19. A raw material gas-gas-steam mixture supply section 20 is positioned beneath the heat transfer bed 17. A discharge section 21 is opened above the steam reforming catalyst bed 16, the discharge section 21 being then connected to a supply section 22 located above the high-temperature reaction section 6. A discharge section 23 for discharging the yielded reformed gas is located beneath the low-temperature shift catalyst bed 9 positioned at the lowermost portion of the inner cylinder 2.
The internal temperature of the high-temperature reaction section 6 is required to be maintained in a high-temperature region so as the steam reforming reaction to be conducted efficiently. To do this, unnecessary thermal diffusion has to be minimized as far as possible. For attaining the minimization of thermal diffusion, a heat-insulation section 24 having a hollow part therein is located between a portion of the inner cylinder 2 where the high-temperature reaction section 6 is positioned and a portion of the outer cylinder 3 where the steam reforming catalyst bed 16 is positioned.
FIG. 14 shows a part-enlarged view containing the heat-insulation section 24. The heat-insulation section 24 has an annular inner wall part 25 and an annular outer wall part 26. By integrating the annular inner wall part 25 with the annular outer wall part 26 at top and bottom, respectively, by the respective side wall parts 27, an annular hollow part 28 is formed inside of them. The inner wall part 25 is a part of the inner cylinder 2.
Next, the method for steam reforming using the above steam reformer 1 is described. When the raw material gas-gas-steam mixture is supplied to the supply section 20, the raw material gas-gas-steam mixture increases the temperature thereof while passing through the heat transfer bed 17, which is in a high temperature state, owing to the heat transfer from the high-temperature shift catalyst bed 8 and the low-temperature shift catalyst bed 9. The raw material gas-steam mixture at a high temperature then enters the steam reforming catalyst bed 16, where a part of the raw material gas-gas is subjected to steam reforming. The reformed gas and the residual raw material gas-gas-steam mixture are discharged from the discharge section 21 in the inner cylinder 2 to enter the high-temperature reaction section 6 via the supply section 22 in the inner cylinder 2.
In the high-temperature reaction section 6, a part of the raw material gas-gas in the influent raw material gas-gas-steam mixture is subjected to oxidation reaction by the oxygen in the oxygen-containing gas supplied from the oxygen-containing gas introduction section 5 in the presence of the oxidation catalyst which structures the mixed catalyst bed 4. The oxidation reaction increases the temperature of the raw material gas-steam mixture to a range necessary for reforming reaction, for example, from about 650° C. to about 750° C., (normally around 700° C.). That is, the self-oxidation and internal heating are carried out. With thus generated heat, the steam reforming reaction of the raw material gas-gas-steam mixture is conducted to yield a hydrogen-rich reformed gas at good efficiency. Specifically, the exothermic oxidation reaction and the endothermic reforming reaction proceed at the same time in the high-temperature reaction section 6, thus the uniform temperature distribution is maintained in the high-temperature reaction section 6. The steam reforming catalyst bed 16 in the outer cylinder 3 functions as a preliminary reforming section for the high-temperature reaction section 6.
The reformed gas yielded in the high-temperature reaction section 6 enters the adjacent section 7 beneath thereof, where the reformed gas decreases the temperature, and passes through the high-temperature shift catalyst bed 8 and then the low-temperature shift catalyst bed 9. During the passage through the high-temperature shift catalyst bed 8 and the low-temperature shift catalyst bed 9, most of carbon monoxide remaining in the reformed gas is converted to hydrogen. The high purity reformed gas leaving the low-temperature shift catalyst bed 9 is discharged from the discharge section 23, and is then supplied to an input device, (not shown), such as fuel cells for vehicles and fuel cells for household power source.
As described above, the heat-insulation section 24 suppresses the heat in the high-temperature reaction section 6 from diffusion to the outer cylinder 3. Nevertheless, as shown by arrow A in FIG. 14, the heat in the high-temperature reaction section 6 diffuses from the inner wall part 25 of the heat-insulation section 24 to the downstream side of the inner cylinder 2, or to the adjacent section 7, and further, a part of the heat also diffuses to the outer wall part 26 of the heat-insulation section 24 from the inner wall part 25 via the side wall part 27. As a result, the thermal energy consumed to heat the high-temperature reaction section 6 increases to lower the thermal efficiency and the reaction efficiency of the steam reformer 1.
Consequently, since only the heat transfer section 24 as shown in FIG. 14 cannot fully suppress the thermal diffusion from the high-temperature reaction section 6 to the adjacent section 7 or to the oxygen-containing gas introduction conduit 4, the effect to lower the thermal efficiency and the reaction efficiency in the steam reformer 1 is insufficient.
To this point, a problem of the present invention is to further solve the thermal diffusion disadvantage at the high-temperature reaction section.
That is, an object of the present invention is to improve the suppression of thermal diffusion from the high-temperature reaction section to the adjacent section.
Another object of the present invention is to improve the suppression of thermal diffusion from the high-temperature reaction section to the oxygen-containing gas introduction conduit.
A further object of the present invention is to improve the suppression of both the thermal diffusion from the high-temperature reaction section to the adjacent section and the thermal diffusion from the high-temperature reaction section to the oxygen-containing gas introduction conduit.
A still another object of the present invention is to improve the suppression of thermal diffusion from the high-temperature reaction section to the adjacent section and to the oxygen-containing gas introduction conduit with a simple structure of the steam reformer.
A still further object of the present invention is to improve the thermal efficiency and the reaction efficiency of the steam reformer.